1. Field of the Invention
The present invention relates to a sheet processing apparatus which performs stapling of a sheet bundle and an image forming apparatus provided therewith.
2. Description of the Related Art
Conventionally, there is the sheet processing apparatus in the image forming apparatus such as a copying machine, a printing machine, and a laser beam printer. In the sheet processing apparatus, for example, the stapling is performed after sheets on which images are already formed are sequentially taken in from an image forming apparatus body. In the sheet processing apparatus, it is necessary that a post-process such as stapling be performed between each sheet bundle job, e.g., at a time interval between the final sheet of the preceding job delivered from the image forming apparatus and the first sheet of the subsequent job. However, when plural positions of the sheet bundle points are stapled, a long time is required to both move a stapler and perform stapling, thereby lengthening a stapling process time. Therefore, it is necessary to lengthen the sheet bundle (job) interval, thereby lowering productivity of the image forming apparatus.
In the case where the distance between sheets delivered from the image forming apparatus is widened at the sheet bundle interval, the productivity is significantly decreased when there is a small number of sheets in the bundle. For example, in the image forming apparatus having the productivity of 100 sheets per minute, the decreases in productivity of two sheets in a sheet bundle and 100 sheets in a sheet bundle when the sheet bundle interval is widened by 0.2 s in the stapling process. The productivity CPM is determined by the following equation. As used herein, the productivity shall mean the number of sheets which can be printed per one minute.
CPM=60/((60/cpm)×a+b)/a)
cpm(copy per minute): image forming apparatus productivity (the number of copies per minute)    a: the number of sheets in bundle    b: stapling process time    In the equation, a stapling process time is added to a total printing time of the image forming apparatus, and the additional value is divided by the number of sheets in the bundle to determine average productivity of the bundle.
As a result of the computation with the equation, the productivity becomes 85.7 cpm in the case of two sheets, and the productivity becomes 99.7 cpm in the case of 100 sheets. The productivity is not significantly influenced by stoppage of the image forming apparatus in the case of 100 sheets while the productivity is largely decreased in the case of two sheets.
Therefore, for example, U.S. Pat. No. 5,020,785 proposes a configuration in which, when the sheet bundle is moved to the next staple position of a stapler, the sheet bundle is relatively moved in an opposite direction to a stapler moving direction along with the movement of the stapler, thereby shortening a time in which the sheet bundle is moved to the next staple position.
However, a weight of the sheet bundle depends on the number of sheets to be stapled and a kind of the sheet. That is, the weight of the sheet bundle is increased as the number of sheets in the bundle is increased or as the size or grammage of the sheets is increased. In a motor for moving the sheet bundle, a rotating speed becomes slower as the weight of the sheet bundle is increased. Therefore, when the sheet bundle is moved at a constant speed irrespective of the number of sheets in the sheet bundle or the kind of the sheet, a motor having the large output is required, which results in upsizing of the apparatus or increasing the cost.